Albert Barnes on John 21:23

Then went this saying, &c. This mistake arose very naturally--

1st. From the words of Jesus, which might be easily misunderstood to mean that he should not die; and,

2nd. It was probably confirmed when it was seen that John survived all the other apostles, had escaped all the dangers of persecution, and was leading a peaceful life at Ephesus. This mistake John deemed it proper to correct before he died, and has thus left on record what Jesus said and what he meant.